1. Field of the Invention
The present invention relates to solder for oxide layer-building metals and alloys. In particular, the present invention concerns low-temperature solder which produces a high-strength, electrically-conductive, metallurgical bond between aluminum and aluminum alloys.
2. Discussion of the Background
Aluminum is relatively cheap, lightweight and, on a weight-for-weight basis, the best electrical conductor at ordinary temperatures. Unfortunately, it is also an oxide layer-building metal: it is highly reactive and when exposed to air quickly forms a surface layer of hard oxide which is electrically nonconductive. Other oxide layer-building metals and alloys include titanium, tantalum and stainless steel.
Although several methods exist for joining aluminum to itself or to other metals, none is very satisfactory for electrical purposes. Mechanical methods, such as bolting or clamping, cannot produce a metallurgical bond. In time, oxidation intrudes into the joint and creates a high-resistance region. This problem is made worse by thermal cycling since the difference in the expansion coefficients of aluminum and other common metals will inevitably loosen any mechanical joint. Welding can metallurgically join aluminum to itself and its alloys but requires elaborate and specialized equipment. For example, to weld aluminum, the aluminum must be brought to its melting temperature of 660.5.degree. C. Also, to prevent fresh oxidation while welding, oxygen from the air must be excluded. Although air exclusion may be accomplished easily in an industrial setting with a flow of inert gas for welding, this technique is not very practical for common "benchtop" use where special atmospheres for soldering are not readily available.
Furthermore, benchtop soldering tools achieve temperatures that are usually no higher than 300.degree. C., too low to cause the aluminum to melt and flow together. Soldering consists of the addition of a second, lower-melting point metal or alloy which has the power to penetrate and lift the surface oxides and wet the aluminum beneath, forming a metallurgical bond. Typically, this occurs only over small portions of the interface, and the joint is likely to perform poorly, both electrically and mechanically. Performance is usually somewhat improved if the oxide layer is mechanically abraded while it is covered by the molten solder.
Solders tried for use on aluminum have usually consisted of tin and zinc in a roughly two-to-one ratio by weight, with or without small amounts of other metals. The other metals added to the tin-zinc solder include aluminum, lead, copper, silver, antimony, arsenic, bismuth and cadmium, among others. While these alloys have some limited ability to penetrate under and lift the oxide coating from the aluminum surface, none performs well at temperatures attainable with benchtop soldering tools. Most efforts to develop an aluminum solder of this type were conducted between the mid-nineteenth century and the 1950's, apparently with little effort expended thereafter.
A solder must have the capacity to form metallurgical bonds with the two base metals that are being joined. The bonding process results in the formation of an alloy characterized by atoms of the solder composition interspersed between atoms of the base metal. The solder must be free to flow yet be capable of bridging gaps or creating small fillets. When the solder is heated to the molten state, it exists as a droplet as a result of the attraction of the molecules within the alloy. If, however, the attraction of its molecules to the base metal equals or exceeds their attraction for each other, the solder easily flows onto, or wets, the surface to which it is being applied.
Germanium, which is found in ores of zinc and silver, and as a trace impurity in coal, is a hard, brittle substance resembling silicon. Its atomic number is 32. Chemically, it is a "metalloid" with properties somewhere between those of metals and nonmetals. While forming a minor ingredient in alloys such as dental gold, germanium did not see widespread use until the late 1940's, when its semiconducting properties were discovered and ushered in the modern age of solid-state electronics. It has since been replaced in nearly all electronics applications by silicon which is less expensive, more easily fabricated, and performs better at high temperatures.
Germanium melts at 938.3.degree. C., well above aluminum, but is easily dissolved by some other molten metals at considerably lower temperatures. High-purity, molten germanium has the peculiar property of wetting and sticking tightly to virtually any substance, metallic or non-metallic. This property appears also in some of its alloys in which germanium acts as a wetting agent.
The need to abrade the surface of oxide-layer forming metals and metal alloys has long been recognized. Methods for preparing metal surfaces usually require removal of this layer before application of the solder. Conti, in U.S. Pat. No. 1,619,852, describes a solder that requires no scraping or cleaning of the surface before soldering. In U.S. Pat. No. 2,733,168, Hodge, et al. describe a solder composition that is sufficiently hard that sticks made of the solder can be used to scrape the metal surface (Column 2, lines 61, et seq.). Riesmayer, in U.S. Pat. No. 2,252,414, discloses the use of a small amount of manganese and/or nickel but specifies that the solder should be free of copper for the solder to penetrate the oxide film. In U.S. Pat. No. 2,552,935, Chadwick uses cerium as a "wetting agent" as well for improved corrosion resistance.
There is a need for a solder which will provide a bond of high mechanical strength and electrical conductivity under common soldering conditions.